Like some reptiles, incubation temperature affects the sex ratio of chicks, however, the mechanism is different between reptiles and these birds, with reptiles exhibiting Temperature-dependent sex determination (TSD), and megapodes exhibiting temperature-dependent embryo mortality.

It is most prevalent and common among amniote vertebrates that are classified under the reptile class,[2] but is absent among birds, including the Australian Brush-turkey, which was formerly thought to exhibit this phenomenon.

It is saying that in reptiles, the sex ratio of the embryos changes depending on the incubation temperatutre. In other words, depending on the temperture, more or less embryos of a particular sex are produced. In the birds, the temperature does not skew the sex ratio, it skews the ratio of embryos that survive and those those don't.

It is saying that in reptiles, the sex ratio of the embryos changes depending on the incubation temperatutre. In other words, depending on the temperture, more or less embryos of a particular sex are produced. In the birds, the temperature does not skew the sex ratio, it skews the ratio of embryos that survive and those those don't.

It's not a "they" that's killing the embryonic chicks; it's just the temperature. When the temperatures are too extreme one way, lots of boy chicks die, and when it's too extreme the other way, lots of girl chicks die.